Best fit FASTWIND model (red, solid line) to sample star 60 782 (black). The chart shows the principal diagnostic spectral lines: H_γ and H_β (gravity), HeII4541 and HeI4471 (T_eff), and HeII4686 (log  Q ). To illustrate that T_eff is well constrained within the error bars, two additional models (nonsolid lines) with T_eff varying ±2000 K are shown. The fit of these two models to the HeII4541 diagnostic line is poor, defining the error bars for temperature.